I. Field of the Invention
This invention relates generally to millimeter wave microstrip transmission lines for transmitting RF energy, and more particularly to an improved energy absorber for fine tuning of the power being transmitted from a source to a load via said transmission line.
II. Discussion of the Prior Art
Microstrip transmission lines are well known in the art. They generally comprise a conductive ground plane upon which is placed a dielectric layer, usually coextensive with the ground plane. Usually centrally positioned atop the dielectric layer is a conductive strip of substantially lesser width than the ground plane. It has been found convenient to fabricate such microstrip transmission lines using well-known printed circuit techniques where one surface of the printed circuit board is metallized to form the ground plane and an elongated strip is etched on the opposed surface of the dielectric printed circuit board. For general information concerning the construction and operation of microstrip transmission lines, reference is made to U.S. Pat. No. 2,654,842 of H. F. Engelmann and to "Proceedings of the I.R.E.", December, 1952, pp. 1644-1650.
In many applications, it is desirable to be able to adjust the amount of RF energy being transmitted from a source to a load. Microstrip attenuators are used for this purpose. Various forms of attenuators for use with microstrip transmission media are also known in the art. These commonly take the form of fixed value, fixed position devices and are designed using standard impedance value models for either T or .pi. configurations. Other somewhat related microstrip attenuators are described in the Arditi et al U.S. Pat. No. 2,890,424 and the Engelmann U.S. Pat. No. 2,810,891. In these latter two patents, a piece of microwave lossy material is positioned in "resilient engagement" with the conductive strip line, and by adjusting the presssure between the lossy material and the strip line, the degree of attenuation can be varied. An undesirable variation of the attenuation, measured in dBs, can occur over time, however. This is believed to be due to either changes in the contact pressure between the attenuating pad and the strip line or in the unwanted deformation (creep) of the lossy material from which the attenuating pads are commonly fabricated due to the application of pressure.